This invention broadly concerns modified polymer compositions and methods for their use in the preparation of polymeric end-products. In particular, the invention relates to a process for covalently cross-linking polyhydroxyalkanoate (PHA) polymer molecules so as to produce branched PHA compositions suitable for use in the production of PHA coatings, films and moldings.
There has been considerable interest in recent years in the use of biodegradable polymers to address concerns over plastic waste accumulation. The potential worldwide market for biodegradable polymers is enormous (&gt;10 B lbs/yr). Some of the markets and applications most amenable to the use of such biopolymers involve those having single, short use applications, including packaging, personal hygiene, garbage bags, and others. These applications, although poorly suited for recycling, are ideally suited for biodegradation through composting.
PHA biopolymers are polyesters produced by numerous microorganisms in response to nutrient limitation. The commercial potential for PHA spans many industries, and is derived primarily from certain advantageous properties which distinguish PHA polymers from petrochemical-derived polymers and other polymers derived from renewable resources, namely excellent biodegradability and/or natural renewability compared to the petrochemical-derived polymers, and hydrophobicity compared to other polymers derived from renewable resources.
Widespread use and acceptance of PHAs, however, has been hindered by difficulties in developing acceptable means by which these biodegradable polymers can be efficiently and economically processed into commercially attractive products. In addition, many characteristics of PHA-derived products, such as ductility, flexibility and elongation, are rapidly lost over time. This "aging" of PHA-derived products is unacceptable for many applications in that the products fail to maintain adequate structural integrity for their intended useful life. Thus, the success of PHA as a viable alternative to both petrochemical-derived polymers and to non-PHA biodegradable polymers, will depend upon novel approaches to overcome the unique difficulties associated with PHA polymer processing and with products derived therefrom.
One important application of biodegradable polymers is in the area of films and coatings for packaging and one-time use applications. Coatings, for example, are typically made by melting a polymeric material and extruding the melt through a die onto a substrate such as paper or board. Extrusion coating of PHA polymers, however, has typically required coextrusion with a non-PHA polymeric sacrificial layer such as polyethylene, resulting in a coated material which comprises a paper layer, a PHA layer, and a sacrificial layer. The use of a sacrificial layer during PHA processing has been necessary to provide sufficient melt stability and adhesion of the PHA coating to the paper surface and to allow the coatings to be processed at commercially desirable line speeds. Typically, the sacrificial layer is subsequently stripped from the PHA coated paper. It is important from a commercial standpoint to develop compositions and methods for cost-effective extrusion operations which do not depend on the use of a sacrificial non-PHA polymeric layer, and which provide PHA films and coatings having good physical and mechanical properties.